Purification of crude terephthalic acid by hydrogenation over a suitable catalyst is well-known. Hydrogenation offers the easiest route for removal of 4-carboxybenzaldehyde (4-CBA) impurity from the crude terephthalic acid (TA). This invention is directed to an improved process for the hydrogenation of crude terephthalic acid in the presence of a catalyst prepared by utilizing palladium metal deposited upon an active carbon support from soluble palladium complexes which react with the carbon to produce a catalyst of improved activity and/or selectivity in hydrogenating 4-carboxybenzaldehyde.
Catalysts comprising a Group VIII metal upon an inert carrier are known for use in various hydrogenation reactions. They are usually prepared by impregnating a support material with a solution of a compound of a Group VIII metal and reducing the impregnated compound to the metal. Catalyst improvements typically have been directed to obtaining increased hydrogenation activity rather than increased activity and/or selectivity in hydrogenating specific compounds.
It is an object of the instant invention to provide an improved method for preparing a supported catalyst of a Group VIII metal. A particular object is to provide a method for preparing such catalysts having increased catalytic activity and/or selectivity in the reduction of 4-carboxybenzaldehyde. Another object is to provide a catalyst composition which comprises crystallites of catalytically active palladium upon the surface of a porous carbonaceous support material wherein a catalyst of improved activity and/or selectivity is obtained for use in a process for reduction of 4-carboxybenzaldehyde in purification of crude terephthalic acid containing up to 10,000 ppm of 4-carboxybenzaldehyde. Still further objects will be apparent from the following specification.
The field of this invention accordingly relates to Group VIII metal catalysts for hydrogenation and purification of terephthalic acid suitable for polyester polymers and copolymers useful in the manufacture of textile fibers. These polymers and copolymers have been made by condensing terephthalic acid with ethylene glycol and other dihydric alcohols.
As with other supported catalysts, the activity and selectivity of a Group VIII metal catalyst upon a carrier depends on numerous factors such as the amount of Group VIII metal or metals present in the catalyst, the type of support, the method by which the Group VIII metal is deposited and the distribution of the metal on the support.
Such Group VIII catalysts are limited in their ability to selectively hydrogenate impurities in the terephthalic acid, especially 4-carboxybenzaldehyde. Users of terephthalic acid, such as textile fiber manufacturers, often put a rigorous limitation on the allowable concentration of 4-carboxybenzaldehyde in terephthalic acid.
Typically, Group VIII metal catalysts, such as palladium catalysts, are prepared by causing a palladium salt to be adsorbed from a solution onto a carrier. In one procedure as is taught in U.S. Pat. No. 2,857,337, the salt is then treated with a water-soluble metal hydroxide or basic carbonate which is thereafter reduced to metallic palladium by reducing agents such as formaldehyde, glucose, hydrazine, glycerine and the like. Other conventional methods of preparing palladium catalysts have been taught. U.S. Pat. No. 2,802,794 teaches impregnation of an activated alumina support material with a solution of a compound of the platinum metal group and reducing the impregnated compound to the metal. The preconditioned activated alumina is obtained by heating a hydrated alumina to a temperature of up to 800.degree. C. whereby a microporous alumina is obtained.
U.S. Pat. No. 3,138,560 to Keith, et al., teaches that when sodium tetrachloropalladate palladium chloride is added to many carbon supports, most of the palladium is immediately deposited as a shiny film of metallic palladium. Catalysts so prepared generally have low activities and it has been theorized that the palladium compound is directly reduced to palladium metal by the presence of functional groups, such as aldehydes or free electrons on the carbon surface. Palladium catalysts are taught accordingly as advantageously prepared by fixing the palladium as an insoluble compound prior to reduction to avoid the problems of migration and crystallite growth which can occur when a metal is reduced from solution. Keith '560 teaches inclusion of an oxidizing agent, such as hydrogen peroxide to hydrolyze the palladium prior to reduction by the carbon, thus obtaining improved palladium dispersion and a highly active catalyst. U.S. Pat. No. 3,288,725 to Aftandilian teaches that catalysts produced by deposition of a transition metal compound upon an inert particulate solid and subsequent reduction often have a disadvantage in that uniform deposition of the transition metal compound upon the surface of the inert particulate is accomplished with great difficulty. Hence, when the metal compound is reduced, the metal atoms deposited on the surface thereof are not exposed, are therefore not completely reduced and maximum potential catalytic activity is not achieved. Aftandilian '725 teaches that reaction of the metal compound with a particulate surface having a suitable hydroxyl group content, followed by reduction with a borohydride produces an improved catalyst. U.S. Pat. No. 3,737,395 to Arnold, et al., teaches a process for preparing a catalyst which avoids formation of gels which cause lower activity. The catalysts are taught as having uniform and controlled deposition of palladium or platinum and a metallic promoter onto particulate carbon. An aqueous slurry is formed of the palladium or platinum compound and the water soluble metallic promoter. A precipitant is then added to precipitate the palladium or platinum and the metallic promoter, followed by coreduction of both with a mild reducing agent such as formaldehyde, hydrazine, sodium formate, glucose or hydrogen. U.S. Pat. No. 3,271,327 to McEvoy, et al., teaches a process for depositing palladium upon the surface of a nonporous support material wherein the palladium forms a thin, firm and adherent coating, thus obtaining maximum catalytic activity by means of a thin, peripheral distribution of palladium oxide in the support material. U.S. Pat. No. 3,328,465 to Spiegler teaches the preparation of palladium metal deposited on nonporous carbon support admixed with a porous carbon. The resulting catalyst is taught as resulting in a rate of hydrogenation about twice that of a hydrogenation process using the same amount of palladium deposited on a nonporous carbon. Previously, carbon used for support of palladium had been mainly porous carbon of vegetable or animal origin. Due to the high porosity of the carbon, some of the palladium became trapped in the pores and did not contribute to the activity of the catalyst. Another disadvantage was that such porous catalysts became fouled with the products of hydrogenation. By dilution of the nonporous carbon with porous carbon, the catalyst metal is distributed throughout the entire carbon without plugging the pores of the porous carbon.
The impurities in crude terephthalic acid prepared from para-xylene are partially oxidized products such as toluic acid and 4-carboxybenzaldehyde. These impurities usually are present in significant amounts. Toluic acid is not a particularly harmful impurity, in that it is readily removed by cooling and crystallizing terephthalic acid solutions containing it. Other impurities, and particularly 4-carboxybenzaldehyde, are more difficult to remove from terephthalic acid as such. Purification of crude terephthalic acid containing a high concentration of 4-carboxybenzaldehyde (4-CBA) is usually accomplished by converting 4-CBA by hydrogenation over a suitable catalyst to products which can be easily separated from the terephthalic acid by crystallization. However, only with great difficulty can the level of 4-CBA be reduced to levels below the limitation required by textile manufacturers. 4-Carboxybenzaldehyde is a particularly undesirable impurity because it acts as a chain-stopper during polyesterification of terephthalic acid.
Accordingly, a catalyst and process are highly desirable whereby impurities in crude terephthalic acid such as 4-carboxybenzaldehyde are hydrogenated to very low levels by selective reduction to readily separable compounds.
A number of techniques and processes have been developed to purify terephthalic acid by hydrogenation using palladium or platinum catalysts conventionally prepared as described above. Various devices are utilized to obtain the desired selectivity and activity in hydrogenating 4-carboxybenzaldehyde.
U.S. Pat. No. 3,522,298 to Bryant, et al., teaches a process wherein crude terephthalic acid is admixed with an inert gaseous carrier such as steam. The vapor mixture is contacted at a temperature of from 600.degree. to 1000.degree. F. with hydrogen in the presence of a catalyst such as a Group VIII metal upon a carbonaceous support, i.e., palladium upon powdered carbon. Vaporized terephthalic acid is separated by condensation from other constituents in the vapor, e.g., steam, other impurities. U.S. Pat. No. 3,542,863 to Zimmerschied teaches that hot formic acid treatment of a palladium metal on charcoal catalyst controls the activity and/or reactivity in instances where initial activity of a fresh catalyst is excessive and causes over-hydrogenation of aromatic rings or carboxylic acid groups or where catalysts have become deactivated due to use with oxygenated hydrocarbons. U.S. Pat. No. 3,584,039 to Meyer teaches purification of terephthalic acid by hydrogenation in aqueous liquid phase upon a Group VIII metal on carbon in the presence of hydrogen followed by crystallization from the mother liquor. U.S. Pat. No. 3,591,629 to Stancell, et al., teaches that a biphenyl treated catalyst of a Group VIII metal on activated carbon particles minimizes the conversion of terephthalic acid in the presence of hydrogen while effecting high conversions of 4-carboxybenzaldehyde contaminating the commercial acid. U.S. Pat. No. 3,607,921 to Stancell teaches that contact of crude terephthalic acid with carbon monoxide in the presence of palladium on carbon support effects a high percentage conversion of 4-carboxybenzaldehyde contaminating the acid. Surface area of the metal on the carbon support is taught as being extremely high, to 120 square meters per gram. U.S. Pat. No. 3,726,915 to Pohlmann teaches that copper based on palladium in palladium/carbon catalysts increases the activity of palladium/carbon catalysts in the hydrogenation of 4-carboxybenzaldehyde. U.S. Pat. No. 3,799,976 to Nienburg, et al., teaches purification of terephthalic acid containing 4-carboxybenzaldehyde by heating an aqueous mixture of the crude acid with formic acid in contact with a Group VIII metal as catalyst. U.S. Pat. No. 4,260,817 to Thompson, et al., teaches a method for purifying crude terephthalic acid by hydrogenating the crude acid to make toluic acid from 4-carboxybenzaldehyde and para-xylene from terephthalyl dialdehyde wherein the reduction takes place in two stages, the aldehyde radical forming an alcohol radical and in turn forming a methyl radical. The catalyst comprises two Group VIII metals on carbon particles.
Accordingly, it is well-known that crude terephthalic acid containing 4-carboxybenzaldehyde and other impurities can be purified by hydrogenation over a Group VIII metal or metals on carbon catalyst. However, more selective catalysts and processes are highly desirable wherein crude terephthalic acid containing high levels of 4-carboxybenzaldehyde is selectively hydrogenated to contain very low levels of 4-carboxybenzaldehyde.